It is common practice to provide rotating machinery with speed governing or speed control devices which are controlled by an rpm-dependent signal from a tachogenerator. Known tachogenerators have a stationary frame or yoke of soft iron forming pole-shoes which carry coils. The rotating portion of the known tachogenerator is a magnet which is attached to the rotating shaft of the machinery. Inasmuch as the shafts of these machines usually rotate in the right-hand sense, it is necessary to use a mounting screw having left-handed threads for attaching the magnet to the shaft in order to prevent its loosening during operation. When service-personnel attempt the removal of the mounting screw during maintenance and repair services, they will often strip the threads of the screw in the attempt to remove it, expecting it to be equipped with the customary right-handed threads. Such damage is especially grave because the magnet assembly in tachogenerators is a calibrated part, uniquely adapted to the particular generator, and cannot usually be readily replaced by another available magnet.